Temperature compensated pile resistor



y 1960 E. H. AYERS TEMPERATURE COMPENSATED PILE RESISTO Filed Jan. 26, 1959 INVEN TOR.

EDGAR H. AYERS ATTY.

United States Patent 2,935,713 TEMPERATURE COMPENSATED PILE RESISTOR Edgar H. Ayers, Battle Creek, Mich., assignor to Clark Equipment Company, a corporation of Michigan This invention relates to resistors of the carbon pile type, and more particularly to a temperature compensated pile resistor.

Carbon pile resistors are well known and have been utilized in regulator apparatus and systems for a relatively long period of time. Known resistors of the type contemplated have utilized a construction wherein a cylindrical steel tube is fixedly connected at one of its ends to a mounting plate or other member, a stack of spring loaded compressible carbon discs are mounted within the tube between the mounted end thereof and an adjustable spring loaded plunger, for example, for varying the compression of the carbon disc pile and therefore the resistance thereof, and a hollow cylindrically shaped insulating member is disposed between the carbon disc pile and the metal casing therefor.

A serious limitation has been found to exist in the use of such resistors for certain applications in which an essentially constant selected resistance, for example, is required, irrespective of variations in temperature of the parts of the resistor assembly. This limitation has resulted from the fact that as the carbon discs of the pile are heated, they expand at a lower rate than does the surroundingmetal casing. Also, it is known that as the carbon discs are heated the resistance of the carbon material decreases.

Heretofore, the manner of mounting such resistor assemblies has been indiscriminate and, to my knowledge, no" consideration has been given previously to the important effect which mounting has on resistance as the temperature of the housing and carbon pile varies. Depending upon the manner of mounting a resistor assembly, an increase in temperature of the carbon pile and the housing therefor may result in either an undesired increase or decrease in resistance of the carbon pile.

I have found that by placing a mounting bracket or equivalent means at a carefully selected location along the length of the metal casing of the pile, the expansion of the casing and of the carbon discs, the negative change in temperature coeificient of the carbon discs, and the consequent change in spring load on the carbon pile may be matched in such a manner as to provide essentially constant resistance of the pile through a wide range of temperature change in the parts of the assembly, or may be matched in a manner to provide a controlled change in such resistance with a change in temperature. Therefore, whether the resistor be operating at a high or a low temperature makes practically no difference since the resistance of the pile may be controlled as aforesaid.

:It is therefore a principal object of my invention to provide a carbon pile resistor wherein the resistance of the pile is controlled throughout a wide range of temperature change.

It is another object of my invention to provide a pile resistor having means selectively movable along the length of the pile for compensating for effects upon the carbon pile and the housing thereof which result from an increase in the temperature thereof, whereby to provide either a controlled variation in the resistance of the pile or a substantially constant resistance thereof, as desired.

2,935,713 fi at ented May 3, 1960 2 I It is a further object of my invention to provide a carbon pile resistor of preselected design which, when combined in a, predetermined manner with mounting means therefor, effects substantially constant resistance of the carbon pile irrespective of variations in temperature of the parts of the resistor assembly.

Other objects and advantages of my invention will become apparent from the following description when taken in conjunction with the accompanying drawing wherein a preferred embodiment of my invention is illustrated:

Figure 1 is a view in perspective of a pile resistor of the type contemplated;

Figure 2 is a sectional view taken lengthwise of the pile resistor shown in Figure 1; and

Figure 3 is a cross-sectional view taken along line 3-3 of Figure 2.

Referring now to the drawing, a cylindrical housing 10 having a plurality of circular cooling fins 12 spaced axially along the length thereof, contains therein a plurality of carbon disc elements 14 which form an axially extending compressible pile within the housing 10. An elongated cylindrical insulating means 16 is positioned between the pile and housing and extends axially thereof to form an electrical insulating sleeve therebetween.

A cover plate 18 having an opening 20 therethrough in axial alignment with the axis of housing 10 is connected to the right end of housing M and a cover end plate 22 of the housing has a similarly axially aligned opening 24 therein.

As shown, an adjustable threaded electrical conductor member 26 projects through opening 24 of housing 10 and is connected at the right end thereof to a cylindrical plate 28 which is actuatable axially of housing 10 by member 26 to vary the compression of the pile 14, and therefore the resistance thereof in a well known manner. As illustrated, member 26 is threadedly connected to an insulating block 29, which is secured to end plate 22. The right hand disc of pile 14 is maintained in abutment with a cylindrical plate member 30 which is connected to the end of a rod 32 extending through the opening 20 and having connected to its other end a nut 34. Formed integral with the nut 34 is a projecting finger 36 which functions as a spring guide for the one end of a spring 38. The spring 38 is mounted between an abutment 40, which may be made adjustable to vary the compression of the spring and thereby the compression of the pile, and the nut 34 for urging with a predetermined force the plate member 30 into holding relation With the pile 14. The projecting end 36 of nut 34 also functions as a stop means for stem 32 so that a predetermined fixed adjustment of the length of spring 38 may be facilitated for any given adjusted position of threaded member 26. The nut 34 is connected to a flexible conductor strap 42 which is registrable'with a groove 44 formed in the periphery of nut 34, and which defines adjacent the top thereof an opening 46 which is connectible with an electric conductor means. Member 26 is similarly connected to a flexible conductor strap 47 which is connected in turn to a conductor, not shown.

A mounting bracket 50 is formed of two rectangularly shaped members 52 and 54 which are each inwardly formed to approximately define a semicircular portion which is engageable with one of the sides of housing 10. The parts 52 and 54 of the bracket member 50 are held together by, for example, bolt means 56 which extend through axially aligned and vertically spaced openings in the bracket members 52 and 54 for engagement with a support and mounting member 58 (Figure 3). Tightening of the bolts 56 will, of course, cause the bracket ice means 50 to tightly engage the periphery of housing for a purpose to be explained.

It has long been a problem in the art to which my invention pertains to devise simple and inexpensive means which functions to effectively compensate the usual effect of variations in temperature on the resistance of a pile of carbon discs such that said resistance is, in the usual case, maintained at a substantially constant value. As is well known, the expansion rate of such a pile is considerably less than the expansion rate of a metal housing, such as steel, in which the pile is mounted; in addition, as above noted, the resistance of carbon discs decreases as the discs are heated. v

The location of mounting bracket 50 will determine, in my construction, what the effect of an increase in temperature will be on the resistance of the carbon pile. For instance, if mounting means 50 is fixedly positioned at the right or spring end of cylindrical housing 10 the effect will be that the housing will expand along its entire length to the left of bracket 54 which will maximize the reduction of pressure of spring 38 on the carbon pile as a result of tr e greater coefficient of expansion of the steel housing, for example, as compared with that of the carbon pile. The spring 33 will expand an amount which is equal to the dilference in expansion of the housing and pile. in such a case, the net result will be that the resulting increase in resistance of the carbon pile (due to the decompression thereof) will be greater than the op posed decrease in resistance of the carbon material due to the increased temperature, thereby effecting a maximum gain in resistance per X degrees increase in temperature of the parts. As a consequence, assuming that the carbon disc assembly is utilized as a rheostat to regulate the speed of an electric motor, the motor speed will be substantially decreased even though regulator member 26, for example, has not been adjusted to effect any such decrease.

On the other hand, should the mounting means 50 be located at the left or fixed end of the tubular housing, as seen in Figure 2, the housing will expand to the right of the bracket along its entire length. Inasmuch as the housing is loosely associated with bolt 32' such expansion will have no effect on the degree of compression of the pile. However, expansion of the carbon discs with such an increase in temperature will result in further compressing same thereby decreasing the resistance thereof. This decrease in resistance due to additional compression will be added to the decrease in resistance resulting from the higher temperature of the carbon material, and a maximum decrease in resistance will result. As a consequence, assuming that the carbon disc assembly is utilized as a rheostat to regulate the speed of an electric motor, the motor speed will be substantially increased even though regulator member 26 has not been adjusted to effect any such increase.

By positioning the mounting bracket between the aforementioned extreme positions, control of resistance of the pile may be effected as desired. In the usual case, as in the control of electric motors, for example, it is desirable to maintain fixed motor speed irrespective of variations in temperature of the controlling means thereof. In such a case a location of the mounting bracket intermediate the right and left end positions may be determined such that effective expansion of housing 10 leftwardly of the bracket position and change of force exerted on the pile 14 by spring 38 will compensate both for differences in the coetficients of expansion of the housing and carbon pile and for variations in resistance of the carbon material resulting from a temperature change to effect an essentially constant resistance irrespective of such temperature change. On the other hand, an intermediate location of the mounting bracket is selectable to effect either a controlled and desired increase or decrease in resistance of the pile, as will be apparent from the foregoing.

Referring now more specifically to the preferred embodiment of my invention, a resistor, assembly suchas illustrated may be controlled to provide a substantially constant resistance irrespective of variations in temperature thereof by properly locating the axial position of the mounting bracket 5i on the housing 10 such that the effective expansion of the housing 10, the expansion of the carbon discs, and the negative change of temperature coeificient of the carbon discs, are matchedto effect the required decrease in the pressure of spring 38. The decrease in the pressure of spring 38 is matched against the decrease in resistance of the carbon pile due to the increase in temperature of the various discs. In other words, since the expansion of the housing 10 to the right of bracket member 5% has no effect upon the pressure applied to the carbon pile 14 by the spring 38 because of the clearance which has been providedbetween the opening 20 in cover plate 18 and the stem 32, the location of bracket member '59 along housing 10' is carefully selected so that that portion of the metal housing 10 to the left of said bracket member expands, with any given increase in temperature, an amount which is greater than the total expansion of the carbon pile. As a result, with plate 28 adjusted to a fixed position, the reduced effective expansion of the carbon pile causes plate 30 to move slightly to the left with insulating block 29 and end plate 22 which decreases the compression of spring 38 and results in a selected decrease in the compression of the carbon pile. In the preferred embodiment, the end result of matching the differential expansion of the housing it) and the carbon discs 14 against the spring 38 must be to effect a resulting decrease in compression of the carbon discs which compensates for a decrease in resistance thereof due to heating so that substantially constant resistance results. Of course, if desired, the mounting bracket may be shifted a selected amount towards the fixed end or the spring end of the resistor assembly from the predetermined constant resistance position thereof to effect a desired decrease or increase, respectively, in the resistance of the carbon pile for a given increase in temperature.

It is assumed, of course, in the examples hereof that the coefficient of expansion of the housing is greater than that of the carbon pile. If such were not the case a different result would be obtained, although the principle of the invention would remain the same.

Although it is possible to approximate by computation the desired location of bracket member 50 for maintain: ing, for instance, a constant resistance of the carbon pile (knowing the coefficients of expansion of the housing material and the carbon discs, the length of the carbon pile, the rate of spring 38, and the decrease in resistance of the carbon discs per degree of temperature rise), an empirical method, or at least a combination of empirical and mathematical methods, is preferred. It has been found that computation alone does not, generally, afford satisfactory results. The primary reason for this relates to the difiiculty in obtaining consistently reliable data which sets forth the actual decrease in resistance of the carbon discs per degree temperature increase, since this factor in itself is a variable which depends upon the level of temperature, disc design factors such as the degree of flatness and surface condition thereof (variations in which result in variations in surface contact area be tween the discs of a pile), and the amount of disc loading.

In order to indicate more specifically the significant result achieved in the meet this invention it may be well to summarize actual data obtained in the use of a particular embodiment thereof, wherein it was the object to maintain constant the resistance of the carbon pile while varying the temperature thereof over a wide range. The data specified below is intended, of course, to be merely illustrative of the significant achievement of the invention and not limitative or restrictive thereof. a

In the use of Allen-Bradley Company size No. 8 carhon pile resistance unit X-18956 equipped with raw carbon discs having a resistance of 2.2 ohms under 2 pounds pressure, it was found that by locating the mounting bracket 6% from the face of the fixed end 22 (or 11 from the face of the spring end 18, the resistor assembly being 18" in length), utilizing a spring 38 having a rate of 60 pounds per inch and a preload thereof on the carbon pile of 3.7 pounds at a room temperature of about 70 F., and then increasing the temperature by applying a direct current through the carbon pile, that an increase in such a temperature from 300 F. to 700 F. (the skin. temperature of the carbon pile housing in the area of highest temperature along the housing length) resulted in a change in the total resistance of the pile of only about 1.7%. By varying said temperature from 100 F. to 800 F., a variation in the total resistance of the pile of only about was obtained.

Prior to my invention, carbon pile resistor assemblies have been used without any special regard for the loca tion of the mounting thereof; in such cases uncontrolled and unwanted variations in the resistance of the pile as high as 300% have been observed as temperature, as aforementioned, was varied between about 100 F. and 800 F.

Of course, design changes or changes in the material used in one or more of the parts of the assembly will generally necessitate a redetermination of the proper location of the mounting bracket to effect a desired result. For instance, a resistor assembly of different length, and/or one utilizing a difierent housing material specification, and/or one utilizing carbon discs of different design, or having a different spring rate and preload, would require alteration of the location of the mounting bracket.

Although only one embodiment of my invention has been specifically illustrated and described, it will be understood by persons skilled in the art that variations in the design, construction and assembly of parts may be made without departing from the scope of the invention.

I claim:

1. A resistor, assembly comprising a substantially cylindrical housing, a pile of resistance elements mounted in said housing and extending longitudinally thereof, abutment means located at opposite ends of said pile, resil ient means urging one of said abutment means into abutment with said pile whereby to effect compression thereof, one end of said housing being inoperatively associated with said one abutment means and said resilient means, and means secured to said housing at a preselected location intermediate the ends thereof for controlling the difference in the degree of expansion of said housing on one side of said latter means and that of said pile upon an increase in the temperature of the housing and pile.

2. A resistor assembly comprising an elognated housing, a pile of resistance elements mounted within said housing and extending longitudinally thereof, adjustment means located adjacent one end of said housing and in abutment with one end of said pile for adjusting the degree of compression thereof, other abutment means adjacent the opposite end of said housing and in abutment with the opposite end of said pile, resilient means urging said other abutment means into abutment with said pile for controlling the resistance thereof, and bracket means secured to said housing at a preselected location intermediate the ends thereof for limiting the amount of expansion of the housing to one side of said bracket means upon an increase in the temperature of the housing.

3. A resistor assembly as claimed in claim 2 wherein said bracket means is secured to said housing at such a location that the difference in the effective amount of expansion between said housing and said pile effects a controlled decompression of said pile upon said increase in temperature.

4. A resistor assembly as claimed in claim 3 wherein the said decompression of said pile effects an increase in the resistance thereof which is substantially equal to the total decrease in the resistance of the elements of said pile upon said increase in temperature.

5. A resistor assembly as claimed in claim 3 wherein said pile of resistance elements comprises a plurality of carbon discs, the resistance of which varies inversely as a function of temperature, and said bracket means is so located on the housing that the decompression of said pile of carbon discs resulting from the differential expansion between the housing and said pile of carbon discs effects an increase in resistance of said pile upon an increase in the temperature of the housing and pile which is substantially equal to the decrease in resistance of the pile resulting from the increase in temperature of the pile.

6. A resistor assembly as claimed in claim 5 wherein that portion of the housing opposite said one side of said bracket means is inoperatively connected to said resilient means and to said other abutment means.

7. A resistor assembly comprising a metal housing having a predetermined coefiicient of expansion, a pile of resistance elements also having a predetermined coefiicient of expansion located within said housing longitudinally thereof, means in abutment with opposite ends of said pile for maintaining same in predetermined compression whereby to effect a determinable resistance of said pile at any given temperature thereof, resilient means urging said abutment means at one end of said pile into abutment therewith for maintaining the compression thereof, one end of said housing being free to move with out alfecting the compression of said pile, and mounting means secured to the housing intermediate the ends thereof, said mounting means being in selected location on said housing such that an increase in temperature-of the housing and pile effects a controlled decrease in the compression of said pile as a result of a greater expansion of that portion of the housing on one side of the mounting means than the expansion of said pile, such differential expansion being controlled to neutralize the tendency of the pile to decrease in resistance upon an increase in the temperature of the pile.

8. A carbon pile resistor assembly comprising an elongated housing having a first coeflicient of expansion, a pile of carbon elements having a second coeflicient of expansion located within and extended longitudinally of said housing, means in abutment with opposite ends of said pile for maintaining same in compression so as to effect a determinable resistance of the pile at any given temperature thereof, yielding means urging said abutment means at one end of said pile into abutment with the pile for establishing the compression thereof, the one end of said housing adjacent said yielding means being free to expand without affecting the compression of said pile, and mounting bracket means secured to the housing intermediate the ends thereof, said bracket means being in a selected location on said housing such that an increase in temperature of the housing and pile effects control of the compression of said pile as a result of the relationship between the expansion of that portion of the housing on 038 side of the bracket means and the expansion of said P er 9. A resistor assembly as claimed in claim 8 wherein the expansion relationship is controlled to effect a decrease in the compression of the pile by virtue of greater expansion of that portion of the housing on said one side of the mounting means than the expansion of said pile.

10. A resistor assembly as claimed in claim 9 wherein said dilference in expansion of the housing and pile is controlled as the result of the selected location of the bracket means on the housing in a manner to compensate the tendency of the carbon elements of the pile to decrease in resistance upon an increase in the temperature thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,191,273 Bradley July 18, 1916 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,935,713 May 3, 1960 Edgar H. Ayers It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 13, after "resistor." insert the following sentence:

The application upon which this patent is granted is a continuation-in-part of my co-pending application entitled "Temperature Compensated Pile Resistor", Serial No. 629,689, filed on December 20, 1956 (now abandoned) Signed and sealed this 4th day of October 1960. (SEAL) Attest:

KARL H, AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 2,935,713 May a, 1960 Edgar H. Ayers It is herebi certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1 line 13, after "resistor, insert the following sentence:

The application upon which this patent is granted is a continuation-in-part of my co-pending application entitled "Temperature Compensated Pile Resistor", Serial No. 629,689, filed on December 20, 1956 (now abandoned) Signed and sealed this 4th day of October 1960. (SEAL) Attest:

KARL AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

